Field of the Invention
The present invention relates to a new and improved microbial growth inhibition test for the determination of the presence or absence of an antibiotic in a sample such as milk.
Description of Related Art
Nowadays, antibiotics are frequently used in veterinary practice not only for treatment of bacterial infections, but also for prophylactic purposes to improve the productivity of foodstuffs. In recent years, this irresponsible misuse of antibiotics as a preventive measure has been a decisive factor favoring the growth of bacterial resistance.
Antibiotic residues are known to be among the most frequently detected contaminants in milk and dairy products and cause important problems in this industrial sector at economical level.
To prevent the negative impact of antibiotic residues on human health and on the entire ecosystem, maximum residue limits (MRLs) for antimicrobials in foodstuff of animal origin have been established by various legislators (such as the EU). The MRL is the maximum concentration of residues of a pharmacologically active substance which may be permitted in food or animal origin.
Microbial growth inhibition tests have been developed for the determination of the presence or absence of antibiotic residues in a sample. Examples of such tests have been described in for example EP 0 005 891A and EP 0 285 792A. The tests described therein are ready-to-use tests that make use of a test organism and an indicator molecule, for instance a pH- and/or redox-indicator. The general principle of the test is that, when an antibiotic is present in a sample in a concentration sufficient to inhibit growth of the test organism, the color of the indicator will stay the same, while, when no inhibition occurs, growth of the test organism is accompanied by the formation of acid or reduced metabolites or other phenomena that will induce an indicator signal.
In general a microbial growth inhibition test has to comply with the basic legislative specifications and reliably identify the presence of antimicrobial residues in the examined samples in concentrations equal to the MRLs. However, microbial growth inhibition tests exist that are unable to detect some antimicrobial residues at their respective MRLs, i.e. they have a limited sensitivity for certain antibiotics. In particular, currently commercialized microbial growth inhibition tests have a limited sensitivity for aminoglycoside antibiotics.
In EP 1 639 122A it has been described that the sensitivity of a microbial growth inhibition test can be increased by using the indicator bromothymol blue. However, the use of this indicator predominantly changes the sensitivity of a microbial growth inhibition test for beta-lactam antibiotics.
In view thereof, there is still a need for a microbial growth inhibition test with an increased sensitivity for, for instance, aminoglycoside antibiotics.